This invention relates to the filtration field, and more particularly, to an improved filtration apparatus for filtering and concentrating a solution.
It is well known that the filtration of fluids may be accomplished through the use of filtration devices which utilize sub-micron filters to filter and concentrate a macro-molecular solution. This technique has been utilized in centrifugal filtration apparatuses which rely on centrifugal forces to direct solutions towards a filter which separates liquid solutions into filtrate and concentrate.
There are certain drawbacks, however, associated with the conventional centrifugal filtration apparatus. Typically, such an apparatus contains a filter unit covering the bottom portion of a housing so that the centrifugal force draws the liquid solution towards the filter unit. One disadvantage with this type of system is that the filtration surface area is relatively small when compared to the volume of liquid solution within the housing. Therefore, filtration occurs at a relatively slow rate. Secondly, the filtration devices contain a semipermeable membrane filter which is conducive to clogging because the more dense molecules within the liquid solution are forced onto the membrane filter.
Another problem with certain conventional filtration devices is their propensity to filter a solution to dryness so that all of the solution will have been filtered. The apparatus continues to run despite all of the concentrate being filtered. Generally, filtration to dryness should be avoided because the recovery and biological feasibility of concentrate is significantly reduced.
Certain types of filtration devices, such as that disclosed in U.S. Pat. No. 4,632,761 to Bowers et al., are capable of preventing filtration to dryness and contain a "dead stop" feature which causes filtration to cease while there is concentrate remaining within the apparatus. The Bowers et al. device, however, filters to dryness when spun at a 90.degree. angle if the device is spun in a swinging bucket centrifuge. Also, in this type of filtration device, the amount of the concentrate remaining after dead stop is dependent upon the angle of rotation of the apparatus. The amount of concentrate remaining in the apparatus is varied by varying the angle at which the apparatus is rotated. However, which this is impossible to do with fixed angle centrifuge rotors. Accordingly, this device will filter to dryness when spun in a swinging bucket centrifuge and cease filtration at only one concentration level when spun in a fixed angle centrifuge.
U.S. Pat. No. 5,112,484 to Zuk depicts a filtration apparatus having a multiple sided filter unit, for separating filtrate from concentrate, which is mounted therein so that the semipermeable membrane is oriented at a direction substantially perpendicular to the centrifugal force applied to the solution to be filtered when the filtration apparatus is used in a centrifuge. In this device, however, the concentrated material becomes increasingly concentrated during the filtration process resulting in the filtration rate progressively declining during use. Moreover, the pressure created by the head of liquid to be filtered decreases as the liquid is filtered through the membrane therefore progressively reducing the filtration rate. During operation of this device, the amount of membrane exposed to the liquid to be filtered diminishes as the fluid is filtered through the membrane thereby also reducing the filtration rate of the device. The pressure head on the membrane is not constant at any given filtration time and the pressure exerted along the membrane varies with the vertical position of the membrane surface.
It is therefore desirable to provide a filtration device which improves filtration rate during operation, minimizes the effects of a diminishing pressure head which results as filtration time increases, operates without variation of the membrane surface area exposed to liquid being filtered and/or filters at a uniform head pressure on the membrane surface.
It is also desirable to provide a filtration device which may be used without filtering to dryness, which has a relatively high filtration membrane surface area thereby enabling filtration to occur at a relatively higher rate, minimizes the clogging of the semi-permeable membrane thereby maximizing filter efficiently, and/or is self-cleaning and, therefore, reusable.
It is also desirable to provide a filtration device which allows dense molecules which may cause fouling to be forced away from the membrane surface by centrifugal force while simultaneously allowing filtrate to flow into a filtrate collection means.